Immature T lymphocytes differentiating in the thymus make gene products which are found in no other normal tissue. These include a DNA polymerase, terminal deoxynucleotidyl transferase (TdT), and, in mice, the glycoprotein TL. Besides appearing transiently in nrmal lymphopoiesis, TdT and TL are produced by many leukemias, sometimes giving phenotypes with no normal analogue. We wish to study the molecular mechanisms that regulate expression of these products in normal development and the ways these are altered in malignant cells. Therefore, we propose to develop recombinant plasmids that will serve as hybridization probes for the RNA species that encode TL and TdT. Using in vitro translation to assay for intact mRNA, we will optimize procedures for enrichment of each message from lymphoma cell RNA. Complementary DNA from these enriched RNA fractions will be cloned in pBR322. We will screen the resulting colonies of transformants by differential hybridization with cDNA made from "positive" and "negative" lymphoma cell RNA, and by positive hybridization-selection of translatable TL and TdT mRNA. The cloned probes will be used on Northern blots to identify TL and TdT mRNAs from tumor cells of different phenotypes and from separated subpopulations of normal thymocytes.